The invention relates to a process for making pressurized multi-component liquid, and more particularly to a process for making pressurized liquid natural gas comprising hydrocarbon components heavier than C5.
Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called xe2x80x9cLNGxe2x80x9d) for transport to market.
The source gas for making LNG is typically obtained from a crude oil well (associated gas) or from a gas well (non-associated gas). Associated gas occurs either as free gas or as gas in solution in crude oil. Although the composition of natural gas varies widely from field to field, the typical gas contains methane (C1) as a major component. The natural gas stream may also typically contain ethane (C2), higher hydrocarbons (C3+), and minor amounts of contaminants such as carbon dioxide (CO2), hydrogen sulfide, nitrogen, dirt, iron sulfide, wax, and crude oil. The solubilities of the contaminants vary with temperature, pressure, and composition. At cryogenic temperatures, CO2, water, other contaminants, and certain heavy molecular weight hydrocarbons can form solids, which can potentially plug flow passages in cryogenic equipment. These potential difficulties can be avoided by removing such contaminants and heavy hydrocarbons.
Commonly used processes for transporting remote gas separate the feed natural gas into its components and then liquefy only certain of these components by cooling them under pressure to produce liquefied natural gas (xe2x80x9cLNGxe2x80x9d) and natural gas liquid (xe2x80x9cNGLxe2x80x9d). Both processes liquefy only a portion of a natural gas feed stream and many valuable remaining components of the gas have to be handled separately at significant expense or have to be otherwise disposed of at the remote area.
In a typical LNG process, substantially all the hydrocarbon components in the natural gas that are heavier than propane (some butane may remain), all xe2x80x9ccondensatesxe2x80x9d (for example, pentanes and heavier molecular weight hydrocarbons) in the gas, and all of the solid-forming components (such as CO2 and H2S) in the gas are removed before the remaining components (e.g. methane, ethane, and propane) are cooled to cryogenic temperature of about xe2x88x92160xc2x0 C. The equipment and compressor horsepower required to achieve these temperatures are considerable, thereby making any LNG system expensive to build and operate at the producing or remote site.
In a NGL process, propane and heavier hydrocarbons are extracted from the natural gas feed stream and are cooled to a low temperature (above about xe2x88x9270xc2x0 C.) while maintaining the cooled components at a pressure above about 100 kPa in storage. One example of a NGL process is disclosed in U.S. Pat. No. 5,325,673 in which a natural gas stream is pre-treated in a scrub column in order to remove freezable (crystallizable) C5+ components. Since NGL is maintained above xe2x88x9240xc2x0 C. while conventional LNG is stored at temperatures of about xe2x88x92160xc2x0 C., the storage facilities used for transporting NGL are substantially different, thereby requiring separate storage facilities for LNG and NGL which can add to overall transportation cost.
Another process for transporting natural gas proposes saturating the natural gas with a liquid organic additive whereby the gas-additive mixture liquefies at a higher temperature than that of the gas alone. For example, in U.S. Pat. No. 4,010,622 (Etter) a natural gas additive is selected from hydrocarbons, alcohols, or esters having a chain length of C5 to C20 and which is liquid at ambient conditions. While the additive-containing natural gas mixture does liquefy at higher temperatures, thereby decreasing the refrigeration costs involved, the process still requires removal of the heavier natural gas components that would be valuable if transported.
It has also been proposed to transport natural gas at temperatures above xe2x88x92112xc2x0 C. (xe2x88x92170xc2x0 F.) and at pressures sufficient for the liquid to be at or below its bubble point temperature. This pressurized liquid natural gas is referred to as xe2x80x9cPLNGxe2x80x9d to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.). Exemplary processes for making PLNG are disclosed in U.S. Pat. No. 5,950,453 (R. R. Bowen et al.); U.S. Pat. No. 5,956,971 (E. T. Cole et al.); U.S. Pat. No. 6,016,665 (E. T. Cole et al.); and U.S. Pat. No. 6,023,942 (E. R. Thomas et al.). Because PLNG typically contains a mixture of low molecular weight hydrocarbons and other substances, the exact bubble point temperature of PLNG is a function of its composition. For most natural gas compositions, the bubble point pressure of the natural gas at temperatures above xe2x88x92112xc2x0 C. will be above about 1,380 kPa (200 psia). One of the advantages of producing and shipping PLNG at a warmer temperature is that PLNG can contain considerably more C5+ components than can be tolerated in most LNG applications.
Depending upon market prices for ethane, propane, butanes, and the heavier hydrocarbons (collectively referred to herein as xe2x80x9cNGL productsxe2x80x9d), it may be economically desirable to transport the NGL products with the PLNG and to sell them as separate products. International patent application published in 1990 under the Patent Cooperation Treaty as WO90/00589 (Brundige) disclosed a process of transporting pressurized liquid heavy gas containing butane and heavier components, including xe2x80x9ccondensiblesxe2x80x9d that are deliberately and intentionally left in the natural gas. In the Brundige process, basically the entire natural gas composition, regardless of its origin or original composition was liquefied without removal of various gas components. This was accomplished by adding to the natural gas an organic conditioner, preferably C2 to C5 hydrocarbons to change the composition of the natural gas and thereby form an altered gas that would be in a liquid state at a selected storage temperature and pressure. Brundige allows the liquefied product to be transported in a single vessel under pressurized conditions at a higher temperature than conventional transportation of LNG. One drawback to the Brundige process is that it does not address handling of heavy hydrocarbons in the natural gas stream that may freeze out at desired temperature and pressure conditions for storage and transportation of the liquefied gas.
In view of the above, it can be readily seen that a continuing need exists for an improved process for making PLNG that retains as much of the entire composition of a natural gas stream as possible, regardless of its origin or original composition, and that minimizes the potential crystallizing of hydrocarbon components at a selected storage temperature and pressure.
The invention relates to a process of manufacturing a pressurized multi-component liquid from a pressurized, multi-component stream, such as natural gas, comprising C5+ components and at least one component of C1, C2, C3, or C4. The process removes from the multi-component stream one or more of the C5+ components and leaves in the multi-component stream at least one C5+ component. The multi-component stream is then liquefied to produce a pressurized liquid substantially free of crystallizable C5+ components at the temperature and pressure conditions of liquid product to be produced from the multi-component stream. In one embodiment, the removal of the one or more C5+ components from the multi-component stream is carried out using a conventional fractionation system that produces a stream lean in the one or more C5+ components and enriched in at least one other C5+ component, which is then liquefied. In another embodiment, one or more of the C5+ components contained in the multi-component gas stream is removed by crystallizing the one or more C5+ components, leaving at least one C5+ component substantially un-crystallized. The crystallized components are separated from the un-crystallized components and the un-crystallized components are liquefied.